Brassica seeds, especially rapeseed, after oil extraction are a potential source of high-quality protein. After oil extraction, rapeseed meal contains about 38% protein compared to approximately 44% in soybean meal, the latter being widely used for feed and food purposes. Proteins contained in rapeseed are rich in lysine and contain adequate quantities of methionine, both of which are limiting amino acids in most cereal and oilseed proteins. However, the use of rapeseed as a protein source in food products has been severely limited as the proteinaceous material which is left over after oil extraction by known methods contains unwanted constituents such as glucosinolates, phenolics, phytates, and hull, which should be removed from the protein meal of these seeds or at least reduced in quantity therein, in order for the meal and the proteins derived therefrom to be acceptable for human consumption.
The reduction or removal of glucosinolates is particularly important, since they are broken down by enzymes present in the seed and in the human body, producing various degradation products that interfere with thyroid function in the body. Thus for human food use, the glucosinolate content of, for example, proteins derived from rapeseed meal should be substantially eliminated to ensure complete product safety. Phenolic compounds impart a bitter favour and dark colour to the final protein products. Phytates are strong chelating agents and affect the utilization of polyvalent metal ions, especially zinc and iron, by strongly binding these metals and making them unavailable for metabolism. The hull is present in large amounts and is indigestible for humans and other monogastric animals. It also gives an unsightly heterogeneous product.
These unwanted constituents are difficult to separate from proteins in the rapeseed meal. Unlike other protein-rich oilseeds such as soybean, peanuts and sunflowerseed, rapeseed has a complex protein composition and contains proteins with widely different isoelectric points and molecular weights. Accordingly, the production of isoelectrically precipitated rapeseed-protein isolates with 90% protein content requires complex processes which result in low yields. Also, the products generally contain high concentrations of phytates, present as protein-phytate complexes. Thus, traditional protein-isolation processes are economically and technically unattractive for the production of high-quality rapeseed proteins.
At present, there seem to be no rapeseed-protein isolates in commercial production. In a recent review article prepared for the Canola Council of Canada, Youngs (see "Technical Status Assessment of Food Proteins from Canola", Canola Council of Canada, October 1985) concluded that, in spite of extensive research, the presence of glucosinolates, phytates, phenolics, and hull still represents a serious problem.
In general, protein isolates from rapeseed material have been extracted experimentally, using multi-solvent co-current or counter-current operations. The extracted proteins are recovered in these operations by precipitation at one or more isolelectric points, sometimes enhanced by heat coagulation or complex formation followed by washing and drying. These experimental processes are too complicated and expensive to be used as viable commercial processes.
Membrane processing by ultrafiltration and/or diafiltration has become a popular laboratory technology for vegetable protein isolation in recent years. Ultrafiltration is a technique for separating dissolved molecules on the basis of their size, shape and flexibility by passing their solution through a membrane which acts as a filter with pore diameters suitable for retaining large molecules. Diafiltration is a special technique of ultrafiltration for the removal of small-molecular-weight compounds from an aqueous solution also containing large molecules.
U.S. Pat. No.4,420,425, which issued to Lawhon on Dec. 13, 1983, explored the potential applications of solubilization and ultrafiltration in soybean and peanut protein systems. Protein extraction of rapeseed was not considered.
In attempting to break the protein-phytate complex to remove the phytate associated with the processing of soybean, peanut, cottonseed and the like, U.S. Pat. No. 3,736,147 dated May 29, 1973, which issued to Iacobucci et al proposed to use diafiltration with 0.2 molar calcium chloride solution at a pH of 3, or phytase in the pH range of 4.5 -7. The process of this patent is directed towards preparing protein products from soybean, cottonseed, peanut, and sesame seed. Protein extraction of rapeseed is not considered.
A rapeseed-protein isolate has been reported by Von Bockelmann et al in "Potential Applications in Food Processing", in the chapter "Reverse Osmosis and Synthetic Membranes", Ed. S. Sourirajan, page 445, National Research Council of Canada, Publication No. NRCC15627, 1977. This isolate contained 30% protein. The isolate produced by Diosady et al (J. Food Sci. 44:768, 1984) contained 80% protein, but was high in phytate.
A rapeseed protein product with a 76% protein content was obtained by Maubois et al using ultrafiltration in U.S. Pat. No. 3,993,636 which issued Nov. 23, 1976. However, a protein content of 76% is too low to be considered an isolate, and since the extraction of the meal was carried out at pH 9, where rapeseed protein solubility is not very high, the protein yield was likely low, although unreported.
Although aqueous sodium hydroxide solutions are effective solvents for rapeseed protein extraction and give high extraction yields, the isoelectric precipitation of these extracts results in low yields, low protein content in the isolates, or both. Likewise, current applications of ultrafiltration and/or diafiltration to rapeseed protein extraction do not result in high-quality rapeseed protein isolates. Thus the problem of developing a commercially viable process for producing pure, food-grade protein from rapeseed up to the present remains unsolved.